The present invention relates to the field of modulators/demodulators (modems) which are electronic circuits intended to transmit digital data on a telephone line or another network.
Telephone lines exhibit distortions of group propagation time and amplitude and therefore, to obtain satisfactory demodulation conditions, these distortions have to be compensated for. This is achieved by means of an equalizer, generally constituted by an auto-adaptive Finite Impulse Response (FIR) filter.
Such equalizers are well known and a specific implementation is, for example, described in U.S. Pat. No. 4,237,554. As indicated in this patent, the coefficients of the self-adaptive filter constituting the equalizer are periodically updated, for example at the rate of bauds, that is, at each symbol arrival. Thus, each coefficient is stored and, at each clock time, the stored value is modified by an adapter which is provided at its input with the present datum and with an error signal issued, for example, by a least square circuit. The least square circuit compares a received symbol, processed by the equalizer, with a stored symbol. In addition, the adapter is associated in the above patent with a tap leakage circuit which decreases the value of the adapted coefficient by a very-small constant value, having the sign of the preceding coefficient. The object of the tap leakage circuit is to get rid of the accumulated rounding errors when calculations are performed in finite arithmetic.
Generally speaking, the known tap leakage processes consist in bringing the coefficients near 0, either by multiplying them by a constant slightly smaller than 1, or by adding a small value, the sign of which is the inverse of the coefficient sign.
Some equalizers simultaneously achieve a channel equalization and a Hilbert transform, thus carrying out, in a single filtering operation, the equalization and the calculation of the complex analytic signal. When these equalizers are used, the quiescent position of the equalizer is no longer a filter essentially formed by null coefficients, but a filter achieving the analytic signal transform. Then, the tap leakage introduces a shift in the desired response. Moreover, in some equalizer structures, at the stationary state, when the equalizer has converged, the signal being of the limited band type, the auto-adaptative algorithm no longer has information on the input signal portions included outside the useful band. Then, a coefficient drift occurs, which is especially high in the case of complex coefficient equalizers and this is liable to introduce high gains outside of the useful band of the signal. This drift can be increased by a tap leakage. A priori, this is not an inconvenient since it occurs outside the useful band but, in fact, the line can be impaired by broad spectrum noises having high energy, such as pulse noises, and these noises will no longer be eliminated by the equalizer.